With respect to an SRS air bag used in a vehicle, a G sensor detects an acceleration of a vehicle at all times, and an acceleration waveform thus achieved is converted to a digital code by an A/D converter and then subjected to operation processing in a computer. When the operation result satisfies a predetermined condition, a power transistor is turned on to supply current to a squib of an air bag module, thereby developing an air bag.
Furthermore, a safe sensor for opening and closing an electrical contact point is provided to a module ignition circuit in series by a mechanical structure, and the contact point is opened at a normal time to interrupt power supply to the module. When suffering an impact exceeding a set level, the contact point is closed and the power supply to the module is turned on.
Furthermore, the power transistor and the safe sensor are disposed while being allocated to the power source side and earth side of the module, thereby forming a “double separating” switch for separating both the power source side and earth side of the module from ECU. Accordingly, even when one side of a vehicle wire to the module is short-circuited to a power supply line or the body, an erroneous development of the air bag can be prevented.
An SRS air bag used in a vehicle is designed so that a self-diagnosis circuit is operated at the start time to detect presence or absence of abnormality of circuits. Since it is desired to detect at least crash decelerations of a vehicle in front-and-rear direction and right-and-left direction, generally, two or more G sensors are provided as disclosed in JP-A-2000-145005, for example. When plural G sensors are provided, it is desirable to provide a self-diagnosis circuit to each G sensor and detect the presence or absence of abnormality of the G sensor by each self-diagnosis circuit.